Interpretive Summary: Use of classical insecticides has introduced severe problems in agricultural and environmental sustainability. Two of most pressing problems are the ability of pest insects to resist classical insecticides and the negative influence of insecticides on ecosystems. One approach to reduce the environmental insecticide load is based on the concept of disrupting insect immune responses to microbial and parasitoid infections. This idea is limited by lack of detailed knowledge of insect immunology and particularly knowledge of how insect immune systems decline with age. To help address this problem, we are investigating how aging influences the ability of adult insects to respond to infection. In this paper we report on the marked decline in immune functions as adult male crickets age. This new research will be directly useful to scientists who are working to improve the usefulness of disrupting insect immune functions as a component of biological control of insects. The ultimate exploitation of insect immunity will benefit a wide range of agricultural producers, and people who rely on agricultural products, by supporting the long-term sustainability of agriculture.

Technical Abstract:
Ecological immunity studies in invertebrates, particularly insects, have generated new insights into trade-offs between immune functions and other physiological parameters. These studies document physiologically-directed reallocations of immune costs to other high-cost areas of physiology. Immunosenescence, recognized as the age-related deterioration of immune functions, is another mechanism of radically altering immune systems. We investigated the hypothesis that aging brings on immunosenescence in adult males of the cricket, Gryllus assimilis. Our data show that the intensity of melanotic nodule formation decreased with adult age from after three weeks post-adult emergence. Circulating hemocyte populations similarly decreased from about 5,000 hemocytes/ul hemolymph to about 1,000 hemocytes/ul hemolymph. The numbers of damaged hemocytes in circulation increased from less than 10% at 1 week post-adult emergence to approximately 60% by three weeks post-adult emergence. The composition of hemocyte types changed with age, with increasing proportions of granulocytes and decreasing proportions of plasmatocytes. The declines in nodule formation were not linked to the adult age of sexual behaviors, which begin shortly after entering adulthood in this species. We infer that age-related senescence, rather than cost reallocations, may account for observed declines in various parameters of immune functions in insects, as seen in other animals.